1. Field of the Invention
The present invention relates to a preparation process of polyhydroxycarboxylic acid which is a degradable polymer used for medical materials and a substitute of general purpose resin, and more particularly, the invention relates to a preparation process of polyhydroxycarboxylic acid from a raw materials containing a certain amount or less of specific impurities.
2. Related Art of the Invention
Polyhydroxycarboxylic acid is excellent in mechanical strength, physical properties and chemical resistance, and additionally has biodegradability which can be harmlessly degraded under natural environment and finally decomposed by microorganisms into carbon dioxide and water. As a result, polyhydroxycarboxylic acid has been particularly focused on as a plastic used for medical materials and a substitute for general purpose resin. It has been known in the preparation of polyhydroxycarboxylic acid that, for example, lactic acid and glycolic acid are usually dimerized by dehydration to obtain a cyclic dimer, and subsequent ring opening polymerization of the cyclic dimer is carried out in the presence of a tin catalyst to obtain a high molecular weight polymer. The process, however, requires complex operation in conducting the reaction and thus the polymer obtained becomes expensive. Additionally, the process is carried out by melt polymerization and the resulting molten polymer is extruded and cut into pellets. Consequently, the product obtained by the process is restricted to pellets. Further, some of the hydroxycarboxylic acids do not form the cyclic dimer and the process cannot be applied.
On the other hand, Japanese Laid-Open Patent SHO 59-096123 and 61-28521 have disclosed processes for preparing polyhydroxycarboxylic acid by dehydrating polycondensation directly from hydroxycarboxylic acid or its oligomers. However, polymers obtained by these processes have an inherent viscosity of about 0.3 dl/g or less, do not have satisfactory mechanical strength and cannot be applied to some uses and objects.
Consequently, polyhydroxycarboxylic acid having sufficient mechanical strength is limited to melt polymerization and the composition of the polymer is also limited.